A typical vehicle safety seat belt system is designed to restrict the displacement of an occupant with respect to the occupant""s seated position within the vehicle when the vehicle experiences a sudden, sharp deceleration. (See U.S. Pat. No. 3,322,163). A typical seat belt has three main portions: the retractor belt, the torso belt, and the lap belt and the performance of each belt may be characterized by its force-displacement curve. The area under the force-displacement curve is referred to as the energy absorbed by the safety restraint.
Current vehicle safety seat belts are made from fully drawn polyethylene terephthalate (xe2x80x9cPETxe2x80x9d) fiber which is partially relaxed (2.7%) and having a tenacity of at least 7.5 grams/denier and 14% elongation at break. U.S. Government regulation requires that seat belts must withstand loads up to 6,000 lbs. However, a problem exists with the current PET fiber based seat belts. Crash studies indicate that after the initial vehicle impact occurs (e.g. at a speed of about 35 miles/hour), the occupant tends to move forward from his seated position until the belt engages to build restraining forces. As indicated in FIG. 1, the relatively unyielding belt made from PET fiber exerts a force of at least 2000 pounds (about 9000 Newtons) against the occupant at the seat belt torso position so as to cause the occupant to have high chest, rib cage, head, neck, and back injuries when the occupant rebounds and impacts the back structure of the seat assembly.
When a car collides at a speed of 35 miles/hour, an impact energy to which an average sized person in the car is subjected is at least 500 Joules on the torso belt. Although the current PET fiber may absorb the impact energy, damage to the vehicle occupant still occurs due to the undesirable fiber force-displacement curve. In 70 milliseconds, an average sized passenger will experience high forces of up to 2,000 pounds (about 9,000 Newtons) as shown in FIG. 1.
In order to absorb the impact energy and to reduce the seat belt load against the vehicle occupant, U.S. Pat. No. 3,550,957 discloses a shoulder harness having stitched doubled sections of the webbing arranged above the shoulder of the occupant so that the stitching permits the webbing to elongate from an initial length toward a final length at a controlled rate under the influence of a predetermined restraining force. However, the stitched sections do not give the desirable amount of energy absorption, do not provide uniform response, and are not reusable in multiple crashes. See also U.S. Pat. No. 4,138,157.
U.S. Pat. No. 3,530,904 discloses a woven fabric which is constructed by weaving two kinds of yarns having relatively different physical properties and demonstrates energy absorption capability. U.S. Pat. Nos. 3,296,062; 3,464,459; 3,756,288; 3,823,748; 3,872,895; 3,926,227; 4,228,829; 5,376,440; and Japanese Patent 4-257336 further disclose webbings which are constructed of multiple kinds of warp yarns having different tenacity and elongations at break. DE 19513259A1 discloses webbings which are constructed of short warp threads which will absorb the initial tensile load acting on the webbing and also longer warp threads which will absorb the subsequent tensile load acting on said webbing.
Those skilled in this technical area have recognized the deficiencies in using at least two different yarn types as taught by the preceding references. U.S. Pat. No. 4,710,423 and Kokai Patent Publication 298209 published on Dec. 1, 1989 (xe2x80x9cPublication 298209xe2x80x9d) teach that when using at least two different yarn types, energy absorption occurs in a stepwise manner and thus, the web does not absorb the energy continuously and smoothly. Therefore, after one type of warps absorbs a portion of the impact energy, and before another type of warps absorbs another portion of the impact energy, the human body is exposed to an undesirable shock.
UK Patent 947,661 discloses a seat belt which undergoes an elongation of greater than or equal to 33 percent when subjected to at least 70% of the breaking load. This reference does not teach or suggest the present load limiting yarn.
U.S. Pat. No. 3,486,791 discloses energy absorbing devices such as a rolled up device which separates a slack section of the belt from the taut body restraining section by clamping means which yield under a predetermined restraining force to gradually feed out the slack section so that the taut section elongates permitting the restrained body to move at a controlled velocity. The reference also describes a device which anchors the belt to the vehicle by an anchor member attached to the belt and embedded in a solid plastic energy absorber. These kinds of mechanical devices are expensive, are not reusable, provide poor energy absorption, and are difficult to control. An improvement on the forgoing devices is taught by U.S. Pat. No. 5,547,143 which describes a load absorbing retractor comprising: a rotating spool or reel, seat belt webbing secured to the reel; and at least one movable bushing, responsive to loads generated during a collision situation, for deforming a portion of the reel and in so doing dissipating a determined amount of the energy. This kind of mechanical device is built-in with a specific amount of load limiting and energy absorption towards certain sized occupants, and cannot be adjusted to the needs of different sized occupants in real transportation scenario. Furthermore, this kind of mechanical device is not reusable to limit the load in multiple crashes since the reel is deformed permanently in the first vehicle collision.
U.S. Pat. No. 4,710,423 and Publication 298209 disclose webbing comprised of relaxed polyethylene terephthalate (xe2x80x9cPETxe2x80x9d) yarns having tenacity of at least 4 grams/denier and an ultimate elongation of from 50% to 80%. Due to the inherent physical properties of PET yarn (e.g. glass transition temperature=75xc2x0 C.), the examples of U.S. Pat. No. 4,710,423 and Publication 298209 show that, at 5% elongation, the load has already reached more than 1500 lbs (about 6,700 Newtons). The damage to the occupant by the seat belt still exists and thus, the belt material needs to be further modified. Examples in these two patents also show that if PET yarn is overrelaxed, the yarn tenacity drops to 2.3 grams/denier.
Kokai Patent Publication 90717 published on Apr. 4, 1995 discloses high strength polybutylene terephthalate homopolymer (xe2x80x9cPBTxe2x80x9d) fiber based energy absorption webbing. The fiber""s tenacity is over 5.8 grams/denier, breaking elongation is over 18.0%, and the stress at 10% elongation is less than 3.0 grams/denier. However, this reference fails to teach PBT fiber demonstrating the initial stress requirement which engages the seat belt to protect the occupant and the means to control the initial stress barrier. A low initial stress barrier of yarn results in a low knuckle force point of the finished seat belt which allows excessive excursion of occupant and leads to serious injuries.
It would be desirable to have an improved energy absorbing seat belt, which has a smoother performance than that of the known stitched webbing approach or the known use of at least two different fibers, is reusable in multiple crashes unlike the known mechanical clamp and device approach, and also addresses the ability to control the initial stress barrier and the impact energy absorption from different sized vehicle occupants.
The present inventors in commonly assigned U.S. patent application Ser. No. 08/788,895 filed Apr. 18, 1997 and allowed U.S. patent application Ser. No. 08/819,931 filed Mar. 18, 1997 have responded to the foregoing need. Also see T. Murphy, xe2x80x9cBuckling Up for the Futurexe2x80x9d, WARD""s Auto World, 95 (1997).
The present invention also responds to the foregoing need in the art by providing load limiting yarn, a process for making the load limiting yarn, and webbing made from the load limiting yarn. The webbing, if used as seat belt to restrain occupant, demonstrates energy absorption and load limiting performance. This type of load limiting seat belt comprises yarn which has a force-displacement profile characterized by:
(a) when the yarn is subjected to an initial stress barrier of from about 0.8 gram/denier to less than or equal to about 1.2 grams/denier, the yarn elongates to less than 5 percent and has an initial modulus in the range from about 30 grams/denier to about 80 grams/denier;
(b) upon subjecting the yarn to greater than the initial stress barrier and to less than or equal to about 1.5 grams/denier, the yarn elongates further to at least about 8 percent; and
(c) upon subjecting the yarn to greater than 1.5 grams/denier, the modulus increases sharply and the yarn elongates further until the yarn breaks at a tensile strength of at least about 6 grams/denier, wherein the yarn comprises a multiplicity of fibers, all of the fibers have substantially the same force-displacement profile, and are made from polymers having a glass transition temperature in the range from about xe2x88x9240xc2x0 C. to about +70xc2x0 C.
The term xe2x80x9cmodulusxe2x80x9d as used herein means the slope of the force-displacement curve.
FIG. 2 illustrates the force-displacement profile of the Inventive Example. The initial stress barrier is indicated as ISB on FIG. 2. The present webbing comprised of this type of yarn is advantageous because the present webbing has better impact energy absorption and a smoother performance than that of the known stitched webbing approach or the known use of at least two different fibers, is reusable unlike the known mechanical device, and also addresses the ability to control the initial stress barrier and the impact energy absorption.
The present invention also provides a process for making load limiting yarn comprising making block copolymer and then spinning the block copolymer into yarn. The present invention for making a block copolymer useful in the present load limiting yarn occurs in a twin screw extruder comprises the steps of:
(A) forwarding aromatic polyester melt to an injection position in the twin screw extruder wherein the aromatic polyester has:
(i) an intrinsic viscosity which is measured in a 60/40 by weight mixture of phenol and tetrachloroethane and is at least about 0.6 deciliter/gram and
(ii) a Newtonian melt viscosity which is calculated to be at least about 7,000 poise at 280xc2x0 C.;
(B) injecting lactone monomer into the molten aromatic polyester from step (A);
(C) dispersing the injected lactone monomer into the aromatic polymer melt so that a uniform mixture forms in less than about thirty seconds; and
(D) reacting the uniform mixture resulting from step (C) at a temperature from about 250xc2x0 C. to about 280xc2x0 C. to form a block copolymer. All of steps (A) to (D) occur in less than about four minutes residence time in the twin screw extruder.
The present process is advantageous because high IV starting aromatic polyester can be used and the short reaction time at high temperature results in block copolymer with minimum transesterification, a high melting point, and a high melt viscosity. Preferably, the block copolymer has a melting point of at least about 220xc2x0 C.
Preferably, the block copolymer melt is then in step (E) devolatilized in the twin screw extruder to remove the residual lactone monomer. Preferably, after the devolatization, ultraviolet absorbers, antioxidants, pigments, and other additives are then in step (F) injected and dispersed into the copolymer melt in the twin screw extruder.
The block copolymer is then forwarded from the twin screw extruder to a fiber spinning equipment. The present process for making fiber from the block copolymer comprises the steps of:
(G) from the twin screw extruder, metering the block copolymer melt at a temperature from about 240xc2x0 C. to about 280xc2x0 C. into a spin pot and extruding filaments from the spin pot;
(H) passing the extruded filaments through a heated sleeve having a temperature from about 200xc2x0 C. to about 300xc2x0 C.;
(I) cooling the filaments with ambient air wherein the air flows perpendicularly to the filament direction at a flow rate of at least about 0.1 meter per second;
(J) applying a spin finish to the cooled filaments;
(K) taking up the filaments to form yarn on a first roll;
(L) passing the yarn to a second roll having a temperature from greater than the yarn""s glass transition temperature to less than the yarn""s crystallization temperature;
(M) drawing the yarn between said second roll and draw rolls over a heated shoe or in a draw point localizer which is positioned between said second roll and draw rolls and has a temperature from about 180xc2x0 C. to about 350xc2x0 C. and then annealing the drawn yarn on said draw rolls having a temperature from about 140xc2x0 C. to about 200xc2x0 C.;
(N) relaxing the drawn yarn between said draw rolls and a final roll so that the relaxed yarn has a shrinkage of about 7 percent to about 20 percent;
(O) cooling the relaxed yarn on the final roll set at room temperature; and
(P) winding up the cooled yarn.
The above process for making block copolymer in a twin screw extruder and spinning load limiting yarn may be carried out in a continuous process from the block copolymerization to the final wound-up yarn or in a discontinuous process where the block copolymer is prepared in the twin screw extruder and chipped and the copolymer chips are then spun from a single screw extruder into load limiting yarn.
Other advantages of the present invention will be apparent from the following description, attached drawings, and attached claims.